Among the various video display systems available in the art, an optical projection system is known to be capable of providing a high quality display in a large scale. In such an optical projection system, light from, a lamp is uniformly illuminated onto an array of, e.g., M.times.N, actuated mirrors, wherein each of the mirrors is coupled with each of the actuators. The actuators may be made of an electrodisplacive material such as a piezoelectric or an electrostrictive material which deforms in response to an electric field applied thereto.
The reflected light beam from each of the mirrors is incident upon an aperture of, e.g., an optical baffle. By applying an electrical signal to each of the actuators, the relative position of each of the mirrors to the incident light beam is altered, thereby causing a deviation in the optical path of the reflected beam from each of the mirrors. As the optical path of each of the reflected beams is varied, the amount of light reflected from each of the mirrors which passes through the aperture is changed, thereby modulating the intensity of the beam. The modulated beams through the aperture are transmitted onto a projection screen via an appropriate optical device such as a projection lens, to thereby display an image thereon.
In FIGS. 1 and 2, there are shown a cross sectional and a perspective views, respectively, of an array 10 of M.times.N thin film actuated mirrors 11 for use in an optical projection system, disclosed in a copending commonly owned application, U.S. Ser. No. 08/340,762, entitled "ARRAY OF THIN FILM ACTUATED MIRRORS FOR USE IN AN OPTICAL PROJECTION SYSTEM AND METHOD FOR THE MANUFACTURE THEREOF", comprising an active matrix 12, an array 13 of M.times.N thin film actuating structures 14, an array 15 of M.times.N supporting members 16 and an array 17 of M.times.N mirror layers 18.
The active matrix 12 includes a substrate 19, an array of M.times.N transistors(not shown) and an array 20 of M.times.N connecting terminals 21. Each of the actuating structures 14 in the array 10 is provided with a first and a second actuating parts 22(a), 22(b), the first and second actuating parts 22(a), 22(b) being identically structured, wherein each of the first and second actuating parts 22(a), 22(b) has at least a thin film layer 23 of a motion-inducing material, e.g., piezoelectric material, including a top and a bottom surfaces 24, 25, an elastic layer 26, and a first and a second electrodes 28, 29. The elastic layer 26 is placed on the bottom surface 25 of the motion-inducing thin film layer 23. The first electrode 28 is located on the top surface 24 of the motion-inducing thin film layer 23, and the second electrode 29 is positioned at bottom of the elastic layer 26, wherein an electrical signal applied across the motion-inducing thin film layer 23 located between the first and second electrodes 28, 29 causes a deformation thereof, and hence the actuating parts 22(a), 22(b). Each of the supporting members 16 is used for holding each of the actuating structures 14 in place and also for electrically connecting each of the actuating structures 14 with the active matrix 12. Each of the mirror layers 18 includes a first side 30, a second opposing side 31, and a center portion 32 therebetween as shown in FIG. 2, wherein the first side 30 and the second opposing side 31 of each of the mirror layers 18 are secured on top of the first and second actuating parts 22(a), 22(b) of each of the actuating structures 14, respectively, such that when the first and second actuating parts 22(a), 22(b) in each of the actuating structures 13 deform in response to the electrical signal, the center portion 32 of the corresponding mirror layer 18 tilts while remaining planar, thereby allowing all of the center portion 32 to reflect the light beams, resulting in an increased optical efficiency.
There are a number of problems associated with the above-described thin film actuated mirror array 10. First of all, each of the actuating structures 14, and hence, the mirror layer 18 attached thereto, has a limited tilting angle. In the array 10, whenever each of the actuating structures 14 tilts upward by more than 3.degree., a part thereof is bound to come in contact with the active matrix 12, thereby limiting the performance thereof. In addition, since the second electrode 29 partially covers the bottom surface 25 of the motion-inducing layer 23 in each of the actuating parts 22(a), 22(b) in each actuating structure 14 to thereby leave portions of the motion-inducing layer 23 in a direct contact with the elastic layer 26 and the respective materials making up the elastic layer 26 and the motion inducing layer 23 are provided with different thermal expansion coefficients, an excessive level of stress may be developed between the motion-inducing layer 23 and the elastic layer 26, leading to an eventual separation thereof, which may, in turn, lead to the formation of cracks or unevenness on each of the mirror layers 18, thereby reducing the optical efficiency of the array 10.